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New Stuff

Haven't posted in a while so just a quick update:

I passed the ONT last month, so I'm now a full blown CCNP!

I'll be taking the CCIE written within the next week

ISCW, Check - Hello ONT

Last week I passed the ISCW with let's just say, a nice margin :)

That means I now have BSCI, BCMSN, and ISCW all passed. ONT, here I come. Eventually.

VLSM Part Three: VLSM, Finally!

Now that we've gone through the nitty gritty details of IP addressing, at last we come to VLSM itself.

VLSM, or Variable Length Subnet Masking, is a method that is very widely used in order to make efficient use of the IP addresses available to us. For example if we have a Class C IP block and a PPP (Peer to Peer) link that will only ever have 2 active hosts, why should we waste all 254 usable addresses on it? That would be a waste! Instead, we manipulate the subnet mask in order to fit the number of hosts that we actually need.

To do this, you need to first know two important formulas:

2^n = The number of available subnets where n is the number of bits used for the network portion of the address.
Ex. With a Class C IP block, a subnet mask of 255.255.255.240 would give us 2^4 = 16 subnets

2^h-2 = The number of usable hosts on a subnet where h is the number of bits used for the host portion. You subtract 2 from the total because the network and broadcast addresses are not usable.
Ex. With a Class C IP block, a subnet mask of 255.255.255.240 would give (2^4)-2 = 14 usable hosts per subnet.

As a sidenote, in IPv6 broadcast addresses have been completely replaced by multicast addresses. What does this mean for us? The formula for usable hosts should now be 2^h-1 for IPv6 since the broadcast address is now usable.

So anyway, back to an example. Now that we know the two all important formulas for VLSM, let's do some theory stuff. Lets say we have a Class C address of 205.4.6.0/24, and we need to be able to use this on our PPP WAN link and addressing the LAN that will have oh, 63 users. For now and for simplicity's sake, we'll start off with the largest subnet which is the LAN with 63 users.

We need at least 61 usable addresses, so let's start plugging in some numbers to our formula, 2^h-2. 5? Nope, that would give us 30. How about 6? That would give us 62 usable addresses which is enough. But what happens if in the future, our LAN grows by 2 devices? We would need to redesign everything, so let's build in some room for growth and use 7 bits of information for the host portion of the address. That means we will use 32 - 7 = 25 bits of the subnet mask for the network portion of the address, which gives us a mask of 255.255.255.128 and 126 usable hosts.

So as we stand this is what we have:
LAN (61 hosts needed):
Network: 205.4.6.0/25
Range: 205.4.6.0 - 205.4.6.127
Usable Hosts: 205.4.6.1 - 205.4.6.126

The next IP address usable for subnetting purposes is 205.4.6.128 since we have already used 0-127 in our first network, so let's start there. On our PPP WAN link, we only need two usable hosts. Let's try using 2 bits for the host portion of the network: 2^h-2 =...........2! Perfect! Since it is a PPP link we don't have to worry about future growth, so having the exact number of hosts needed is perfectly fine. That means for this network, we will be using 32 - 2 = 30 bits of the subnet mask for the network portion of the address, which gives us a mask of 255.255.255.252 with 2 usable hosts.

So finally, we end up with this:
LAN (61 hosts needed):
Network: 205.4.6.0/25
Range: 205.4.6.0 - 205.4.6.127
Usable Hosts: 205.4.6.1 - 205.4.6.126

WAN (2 hosts needed):
Network: 205.4.6.128/30
Range 205.4.6.128 - 205.4.6.131
Usable Hosts: 205.4.6.129 - 205.4.6.130

And we're done! We even still have the rest of the Class C range from 205.4.6.131 - 205.4.6.255 to dedicate to future growth if needed.

Woot!

Sorry for not getting back to VLSM for a while (for the half a person that reads this maybe!). I just finished writing a 50 page research syntax for my Internetworking II class that covers the BSCI and BCMSN material. 15 Topologies with full configurations, command descriptions, and test cases for each of them.

Damn.

VLSM Part Two: Subnetting

The second part to understanding VLSM is knowing exactly what a subnet does and how they are made.

A subnet mask is another binary 32 bit value with 4 octets that can split a network into several sub-networks, hence the name. There are three important values that we have to keep in mind when working with subnets; the network address, the broadcast address, and the range of usable host addresses.

The network address is the address that like it sounds, determines what network any given IP address is on. It is also the value that will be used to base routing decisions on. In order to determine a network address from an IP address, you take the binary values of both the IP address and the subnet masks and do a binary AND comparison. Lets use an example we are familiar with from my last post, so let's use an IP address of 192.168.0.128 with a subnet mask of 255.255.255.0.

When we do a binary AND comparison, there are 3 possible combinations:
1 AND 1 = 1, 1 AND 0 = 0, 0 AND 1 = 0.
This means that all we are concerned about at the moment is where both values are 1.

IP Address:     11000000.10101000.00000000.10000000
Subnet Mask:    11111111.11111111.11111111.00000000

Network Address:11000000.10101000.00000000.00000000

Translate that back into our standard format, and we see that the network address is 192.168.0.0!

Now, let's move onto the broadcast address, which is the address used in order to send a packet to every single address on a given network. In order to figure out the broadcast address, we take the subnet mask and invert all of the host bits and tally them up. Let's use the example above.

We are using the 24 bit subnet mask of 255.255.255.0, so the first 24 bits are being used for subnetting.

Subnet Mask:          11111111.11111111.11111111.00000000
Inverted Host Bits:   11111111.11111111.11111111.11111111

That works out to be 255 when converted back to standard form. All we have to do is append that onto the network portion of the address, and we get 192.168.0.255!

Once you know those two values, finding the usable host addresses is easy. All you do is take the range between the network address and the broadcast address, and that's it!

So in our example, the network address is 192.168.0.0, the broadcast address is 192.168.0.255. The usable ip range is then 192.168.0.1 - 192.168.0.254.

We're getting to VLSM soon, I promise!

VLSM Part One: IP Addresses

The first part to understanding VLSM is understanding IP addressing itself.

An IP Address is an identifier for a network interface whether it's on a PC or a networking device. The IP address is actually a 32 bit binary series divided into 4 octets. The way we're used to looking at IP addresses is just for ease of use and readability. Let's take a look at an example:

Let's take 192.168.0.1/24 and break it down. The /24 on the right refers to bit notation by the way, meaning 24 bits are being used for subnetting. We'll get to that in a little.

In binary, we would have 11000000.10101000.00000000.0000000 for the IP address. Each one of those binary slots represents 1 bit of information.

Now, let's go back to the subnet. Remember how I said 24 bits were being used for subnetting? That means the first 24 bits of the subnet mask are all '1', meaning they're all part of the network portion of the address. In binary, that would be 11111111.11111111.11111111.00000000, which translates to 255.255.255.0.

That number probably sounds pretty similar to a number you usually see as your subnet mask on your PC or router. That's because most routers default to the RFC1918 Class C private network. 192.168.0.0/24.

Anyway, now we have the IP address and subnet mask all converted to binary, so what now? Guess you'll have to wait for my next post about subnetting.